1. Field of the Invention
The present invention relates to a method and a device for detecting failure in a fuel supply system of an internal combustion.
2. Description of the Related Art
A failure detecting device which is capable of detecting failure of elements in a fuel supply system of an internal combustion engine such as an air-flow meter and a fuel injection valve based on an output signal of an air-fuel ratio sensor disposed in an exhaust gas is commonly used. A failure detecting device of this type is disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 5-163983. The device in the '983 publication sets the amount of fuel TAU which is supplied to the engine based on an air-fuel ratio feedback correction factor FAF and a feedback learning correction factor FGHAC using the following formula. EQU TAU=TP.times.(FAF+FGHAC).times.T.sub.1 +T.sub.2 ( 1)
TP in the above formula is a basic fuel supply amount which is required to maintain an operation air-fuel ratio of the engine at a stoichiometric air-fuel ratio, an T.sub.1 and T.sub.2 are predetermined constants determined by the operating conditions of the engine. The air-fuel ratio feedback correction factor FAF is calculated in accordance with the output signal of the air-fuel ratio sensor in such a manner that FAF is increased when the air-fuel ratio of the exhaust gas is higher than the stoichiometric air-fuel ratio (i.e., when the air-fuel ratio of the exhaust gas is lean) and decreased when the air-fuel ratio of the exhaust gas is lower than the stoichiometric air-fuel ratio (i.e., when the air-fuel ratio of the exhaust gas is rich). The feedback learning correction factor FGHAC is a correction factor which is determined by a learning control which will be explained later in detail in such a manner that the center value of the fluctuation of the air-fuel ratio feedback correction factor FAF agrees with a reference value (for example, 1.0).
When the characteristics of the elements in the fuel supply system such as an airflow meter and a fuel injection valve agree with design characteristics, i.e., when there is no change in the characteristics due to a lapse of time, or inherent individual deviations of the characteristics, the value of the air-fuel ratio feedback correction factor FAF always fluctuates around a center value of 1.0 when the air-fuel ratio of the engine is feedback controlled in accordance with the output of the air-fuel ratio sensor. In this case, since the value of the feedback learning correction factor FGHAC is changed so that the center value of the fluctuation of FAF agrees with the reference value 1.0, the value of FGHAC always becomes 0. Namely, if the characteristics of the elements in the fuel supply system do not deviate from the design characteristics, the value of the feedback learning correction factor FGHAC always becomes 0. Therefore, the value of the term (FAF+FGHAC) in the above formula (1) also fluctuate around the center value 1.0.
However, if one of the characteristics of the elements in the fuel supply system deviates from the design characteristics due to, for example, a lapse of time, the center value of the fluctuation of FAF also deviates from the reference value of 1.0. For example, if the amount of fuel supplied to the engine becomes larger than a designed value due to a change in the characteristics of an element in the fuel supply system, the air-fuel ratio of the exhaust gas becomes rich, and the air-fuel ratio sensor outputs a rich air-fuel ratio signal. First, this causes a decrease in air-fuel ratio feedback correction factor FAF and, thereby causes FAF to fluctuate around a center value less than 1.0 to reduce the amount of fuel supplied to engine. Assuming that the value of FAF starts to fluctuate around the center value (1.0-.alpha.), since the value of the feedback learning correction factor FGHAC is maintained at 0 at the start of the deviation of the characteristics of the element, the value of the term (FAF+FGHAC) in the above formula (1) also fluctuates around the center value (1.0-.alpha.). However, since the value of FGHAC is adjusted by a learning control in such a manner that the center value of the fluctuation of FAF agrees to 1.0, the value of FGHAC gradually decreases to a value which makes the center value of the fluctuation of FAF agree with 1.0 (i.e., the value of FGHAC decreases to -.alpha. from 0 after a certain time has elapsed). Thus, the center value of the fluctuation of FAF returns to 1.0 while maintaining the center value of the fluctuation of (FAF+FGHAC) at (1.0-.alpha.) after a certain time has elapsed since the characteristics of the element deviated from the design characteristics. Therefore, the fuel supply amount is reduced to correct the deviation of the characteristics of the element while maintaining the center value of the fluctuation of FAF at the reference value 1.0.
Similarly, if the amount of fuel supplied to the engine becomes smaller than the designed value due to change in the characteristics of the element in the fuel supply system, the value of FGHAC increases to increase the fuel supply amount while maintaining the center value of the fluctuation of FAF at the reference value 1.0. Namely, the value of the feedback learning correction factor FGHAC changes in accordance with the change in the characteristics of the elements in the fuel supply system. By this learning control using the factor FGHAC, the air-fuel ratio of the exhaust gas (i.e., the operating air-fuel ratio of the engine) is maintained at the stoichiometric air-fuel ratio while maintaining the center value of the fluctuation of FAF at the reference value even when the characteristics of the elements in the fuel supply system deviate from the design characteristics.
As explained above, in the '983 publication, two types of correction factors, i.e., an air-fuel ratio feedback correction factor FAF and a feedback learning correction factor FGHAC are used to control the air-fuel ratio of the engine. The air-fuel ratio feedback correction factor FAF is used for correcting a temporary change in the air-fuel ratio caused, for example, by the change in the operating conditions of the engine, and the value of FAF changes quickly in accordance with the change in the air-fuel ratio. The feedback learning correction factor FGHAC is used for correcting a permanent change in the air-fuel ratio caused, for example, by the change in the characteristics of the elements in the fuel supply system, and the value of FAF changes gradually in accordance with the change in the value of FAF. As a result, the value (FAF+FGHAC) always indicates whether failure has occurred in the fuel supply system. For example, when a fuel injection valve of the engine fails and the amount of fuel injection suddenly increases, the value of FAF largely decreases in a short time to reduce the fuel injection amount. This causes the value (FAF+FGHAC) to decrease in a short time after the fuel injection valve has failed. Then, the value of FGHAC decrease gradually, and the value of FAF gradually increases until it returns to the reference value 1.0. However, even during the changes in the values of FAF and FGHAC, the center value of the fluctuation of (FAF+FGHAC) is maintained at a constant value much smaller than 1.0 in this case. Similarly, if the fuel supply amount suddenly decreases due to failure in the fuel supply system, the center value of the fluctuation of (FAF+FGHAC) becomes a value much larger than 1.0 from the instant when the failure occurs. Therefore, it is considered that failure occurs in the fuel supply system if the value of (FAF+FGHAC) fluctuates beyond the range of the fluctuation normally caused by the deviations of the characteristics of elements.
However, in the engine equipped with an evaporative emission control device in which the fuel vapor from a fuel tank is supplied to an intake air passage of the engine to prevent evaporative emission, a problem arises if the failure in the fuel supply system is detected based on the value of (FAF+FGHAC). In this engine, the fuel vapor from the fuel supply system is supplied to the engine in addition to the fuel injected from the fuel injection valves. Therefore, since a total amount of fuel supplied to the engine is increased when the fuel vapor is supplied to the engine, the value of (FAF+FGHAC) becomes a smaller value compared to the value when the fuel vapor is not supplied to the engine even if failure does not occur in the fuel supply system, and if failure is detected based on the value of (FAF+FGHAC), error in the failure detection may occur.
In the '983 publication, this problem is solved by the following method. Namely, the failure detecting device in the '983 publication, determines that failure occurs in the fuel supply system when the value of (FAF+FGHAC) becomes smaller than a predetermined lower limit value. However, when the fuel vapor is supplied to the engine, the device in the '983 publication does not determine the failure immediately even if the value (FAF+FGHAC) becomes smaller than the lower limit value. In this case, the device stops the fuel vapor supply to the engine and sets the value of the feedback learning correction factor FGHAC to 0, and after a predetermined time has elapsed, determines whether the value of (FAF+FGHAC) is lower than a predetermined lower limit. The device determines that the fuel supply system has failed only when the value of (FAF+FGHAC) is still lower than the lower limit when the predetermined time has elapsed after the fuel vapor supply has been stopped. If there is no failure in the fuel supply system, the center value of the fluctuation of (FAF+FGHAC) gradually converges to the original value corresponding to the deviation of the characteristics of the elements in the fuel supply system after the fuel vapor supply to the engine has been stopped. Therefore, by determining failure in the fuel supply system in this condition, an error in the failure detection due to the fuel vapor supply is eliminated.
However, further problems may arise in the failure detecting device of the '983 publication. Namely, in the '983 publication, the center value of the fluctuation of FAF is adjusted by a learning control using only the feedback learning correction factor FGHAC regardless of whether the fuel vapor is supplied to the engine. As explained before, the feedback learning correction factor FGHAC was originally intended to compensate for the change in the characteristics of the elements in the fuel supply system and the value of FGHAC changes at relatively low speed. However, in the '983 publication, the same feedback learning correction factor FGHAC is used for compensating for the fuel vapor supplied to the engine, in addition to the change in the characteristics of the elements. In the '983 publication, when the fuel vapor supply to the engine is started or stopped, the center value of the fluctuation of FAF deviates largely from the reference value 1.0 since the amount of fuel supplied to the engine changes in accordance with start and stop of the fuel vapor supply. This deviation of the center value of FAF is corrected by the change in the value of FGHAC. However, since the changing speed of the value of FGHAC is relatively slow, it takes a relatively long time before the center value of FAF converges to 1.0. Therefore, in the '983 publication, every time when the fuel vapor supply is started or stopped, the center value of FAF deviates from 1.0 for a relatively long time. Further, in the '983 publication, the value of FGHAC is reset to 0 every time when the fuel vapor supply is stopped to perform the failure detection. This causes the center value of FAF to deviate, by a large amount, from 1.0 every time the failure detection is carried out. As explained later, when the center value of FAF deviates from the reference value 1.0, the controllable range of the air-fuel ratio of the engine becomes narrow. Therefore, in the '983 publication, when the failure detection is carried out, the controllable range of air-fuel ratio of the engine becomes narrow for a relatively long time.
Further, according to the device in the '983 publication, it is difficult to correctly detect the failure of fuel supply system in which the fuel supply amount to the engine decreases. For example, if the fuel injection amount of the fuel injection valve decreases due to, for example, blockage of injection nozzle by carbon deposit, the value (FAF+FGHAC) increases by a large amount to compensate for the decrease in the fuel injection amount. However, if this failure occurs when the fuel vapor is supplied to the engine, the amount of increase in the value (FAF+FGHAC) becomes smaller since the fuel vapor is supplied to the engine. In this case, the value (FAF+FGHAC) may stay lower than the upper limit value. In the '983 publication, when the value (FAF+FGHAC) is lower than the upper limit value during the fuel vapor supply, it is determined that the fuel supply system is normal even if the system has actually failed. In fact, the device in the '983 publication is directed only to the detection of the failure of the fuel supply system in which the fuel supply amount to the engine increases (i.e., the failure in which the value (FAF+FGHAC) becomes lower than the lower limit) in order to prevent the error in the failure detection.